Method for production of a steel tubular product, in particular an airbag tubular product, and a steel tubular product produced using this method, in particular an airbag tubular product

ABSTRACT

The invention concerns a method for production of a steel tubular product ( 1 ), in particular an airbag tubular product, with the following steps:
         a) provision of a steel tube ( 2 ),   b) shaping of the steel tube ( 2 ) into a pre-geometry ( 3 ), wherein in an end region ( 4 ), an outer diameter ( 5 ) of the steel tube ( 2 ) is reduced by axial movement into an outer tool,   c) calibration of an inner diameter ( 7 ) of the pre-geometry ( 3 ), wherein the pre-geometry ( 3 ) is still laid in the outer tool, and an inner mandrel, with an outer diameter corresponding to the inner diameter ( 7 ) of the calibrated pre-geometry ( 3 ), is introduced into the end region ( 4 ) of the pre-geometry ( 3 ), and the pre-geometry ( 3 ) is pressed against the outer tool such that the inner diameter ( 7 ) of the pre-geometry ( 3 ) is calibrated by shaping,   d) removal of the pre-geometry ( 3 ) from the outer tool ( 5 ) and removal of the inner mandrel from the pre-geometry ( 3 ),   e) axial movement of the pre-geometry ( 3 ) into a drawing tool with a roll-in contour having a pot-like concavity, with simultaneous shaping of the pre-geometry ( 3 ) into the tubular product ( 1 ) with a rotationally symmetrical outlet opening ( 8 ) positioned centrally in the end face,   f) removal of the tubular product ( 1 ) from the drawing tool.

The invention concerns a method for production of a steel tubular product, in particular an airbag tubular product, and a steel tubular product produced using this method, in particular an airbag tubular product. High-strength or ultra-high strength steel tubular products are used in many technical industrial applications. In particular, such tubular products are also used as airbag tubes.

DE 197 14 753 B4 describes a method for production of such a tubular product. A tubular metal workpiece is introduced into a mold, the diameter of which is substantially greater than that of the workpiece, wherein the mold of the workpiece is driven in rotation by separate drives in the same rotational direction around mutually parallel aligned longitudinal axes, and a shaping pressure is generated by an advance movement of the mold and workpiece. During this shaping, the workpiece rolls on the periphery of the mold, and the end of the workpiece in the mold is melted by frictional heat and formed into a container base. A mandrel is introduced into the workpiece and, during shaping, exerts an internal pressure on the container being formed, and rotates with the workpiece.

Such flow rolling is based on the process principle of pressing or pressure rolling. The essential difference from this is that the tool is not a roller which starts from a specific shape, but a pot-like tool in which the entire mold course is worked. The components which can be produced using this method must however be rotationally symmetrical. Such a method is described in detail under the following link: httpsJ/gfu-forming.de/technologien-umformmaschinen/fliessrollieren/

DE 196 07 010 C1 also describes such a method.

Although satisfactory results for tubular products, in particular airbag tubular products, can be obtained using this method, such production processes have considerable disadvantages. Because of the rotation of the tubular product and also of the tool during the production of the tubular product, complex equipment is necessary for producing such tubular products. Also, production is very time-consuming and the tube surface may be adversely affected.

It is therefore an object of the invention to provide a simpler production method for such tubular products, in particular airbag tubular products, in order to significantly reduce the complexity of manufacturing and equipment.

This object is achieved by a method with all features of claim 1. Advantageous embodiments of the invention are described in the subclaims.

The method according to the invention for production of a steel tubular product, in particular an airbag tubular product, comprises the following steps:

-   -   a) provision of a steel tube,     -   b) shaping of the steel tube into a pre-geometry, wherein in an         end region, an outer diameter of the steel tube is reduced by         axial movement into an outer tool,     -   c) calibration of an inner diameter of the pre-geometry, wherein         the pre-geometry is still laid in the outer tool, and an inner         mandrel, with an outer diameter corresponding to the inner         diameter of the calibrated pre-geometry, is introduced into the         end region of the pre-geometry, and the pre-geometry is pressed         against the outer tool, such that the inner diameter of the         pre-geometry is calibrated by shaping,     -   d) removal of the pre-geometry from the outer tool and removal         of the inner mandrel from the pre-geometry,     -   e) axial movement of the pre-geometry into a drawing tool with a         roll-in contour having a pot-like concavity, with simultaneous         shaping of the pre-geometry into the tubular product with a         rotationally symmetrical outlet opening positioned centrally in         the end face,     -   f) removal of the tubular product from the drawing tool.

With the method according to the invention, the complexity of manufacturing and equipment for production of a tubular product according to the invention, or an airbag tubular product, is significantly reduced since, in comparison with production of such tubular products by means of flow rolling, rotation of both the workpiece and also of the tool can be completely omitted. The fact that only axial movements of the tubular product and/or the tools used are required does not only significantly reduce the complexity of manufacturing and equipment. Rather, in this way the process of shaping the provided steel tube can take place more quickly and economically. Furthermore, by avoiding the rotation of the steel tube and/or tool, very high friction temperatures can also be avoided, so that by reducing the shaping temperatures, the formation of scale can be significantly reduced. Also, with the multistage shaping and calibration process according to the invention, the complexity of material removal operations during production of the tubular product can be minimized, wherein material removal for the outlet opening even becomes obsolete.

Calibration of the inner diameter of the pre-geometry as described in step c) means that, in comparison with flow rolling known from the prior art, a reduction in shaping temperature is achieved which contributes to a substantial reduction in scale formation. This shaping preferably takes place as cold forming or cold drawing.

In a first advantageous embodiment of the method according to the invention, it is provided that the shaping in step b) takes place as cold forming or cold drawing. In comparison with flow rolling known from the prior art, this measure too achieves a reduction in shaping temperature, which contributes to a considerable reduction in scale formation.

In cold drawing or cold forming, shaping takes place at a room temperature of around 293 K and a temperature of less than 473 K, in particular without preheating and without heated tools. The omission of preheating of the workpiece or tools achieves a significantly better energy balance, since the workpieces and/or the tools must be preheated for flow rolling.

The drawing and calibration in step b), and the above-described following calibration before step c), are carried out over a partial length of the tube or tube end. In one embodiment of the invention, this partial length may be completely finish-formed by the rolling in, such that no portion of reduced diameter remains on the finished tubular product.

It is however also possible that, in targeted fashion, a greater partial length is drawn in and calibrated than is used or consumed in the subsequent rolling in, so that a partial length with reduced diameter, calibrated to maximum size, remains between the rolled tube end and the unshaped tube portions. This partial length can preferably be used as a fixing region for inserts and/or attachments on the tubular product.

According to a further concept of the invention, it is provided that the shaping in step d) takes place as hot forming or semi-hot forming. With a steel alloy, this means preheating the workpiece, i.e. here the pre-geometry, to a temperature range between 673 K and the Ac3 temperature (around 1173 K for steel alloys)+50 K, whereby easier shaping of the pre-geometry is achieved. In the case of hot forming, preheating takes place to at least the Ac3 temperature for the purpose of austenitizing the steel alloy, and after step d), or immediately after the optional finish-forming described in the following paragraph, quenching is carried out for the purpose of hardening.

According to a particularly advantageous embodiment of the method according to the invention, it is provided that a calibration or a finish-forming of the end region and outlet opening of the tubular product takes place by insertion of a second inner mandrel, which has an outer contour corresponding to the inner contour of the end region to be produced of the tubular product, and an axial movement of the second inner mandrel and tubular product into a second outer tool, the inner contour of which corresponds to the outer contour of the end region to be produced of the tubular product. This embodiment of the invention achieves, in a simple fashion, that the end region of the tubular product is easily shaped by axial movement of tools or inner mandrels and/or the pre-geometry, without rotation of the pre-geometry or one of the tools and/or inner mandrel. Here too, the complexity of manufacturing and equipment is significantly minimized with the method according to the invention.

It has proved particularly advantageous that for calibration or finish-forming of the outlet opening, a second inner mandrel is used with a rotationally symmetrical element positioned centrally in the end face, the outer contour of which element corresponds to the inner contour of the outlet opening of the tubular product to be produced. In this way, in simple fashion, it is ensured that not only the entire end region of the tubular product can be produced with substantially reduced complexity of manufacturing and equipment, without rotation of the tubular product or pre-geometry or one of the tools including inner mandrels.

In order to optimize the energy balance of the production method according to the invention, it is provided that the calibration or finish-forming of the end region and/or the outlet opening takes place in a residual heat of a hot forming or semi-hot forming carried out in step d), in particular at a temperature of at least 473 K of the used steel or used steel alloy. Preferably, after step d), calibration or finish-forming of the end region and outlet opening is carried out immediately. The shapability during finish-forming of the tubular product to be produced is better, the higher the residual heat during calibration or finish-forming of the end region and/or outlet opening. Also, an immediate finish-forming using residual heat allows hardening if heated before step d) to >Ac3 temperature and the tubular product is rapidly cooled or quenched immediately after finish-forming.

In order to carry out the shaping of the steel tube or pre-geometry with minimal friction, it is provided that for shaping, lubricants are introduced between the steel tube or pre-geometry and the tools or inner mandrels. At least for hot forming or semi-hot forming, preferably a temperature-resistant lubricant is provided.

According to a particular concept of the invention, it is provided that as steel for the steel tube, a steel alloy and in particular a hardenable UHS steel is used. Such steels offer the necessary stability so that tubular products produced in the method according to the invention can also withstand high pressures, such as may occur for example in their interior when such tubular products are used as airbag tubular products.

In a particular concept of the invention, the method according to the invention is therefore distinguished in that, as a material of the tube or tubular product to be produced, a steel is used which, as well as iron and unavoidable melt-induced contaminants, comprises the following alloy elements as percentage by weight:

-   -   C 0.07 to 0.50, preferably 0.07 to 0.20;     -   Si 0.05-0.55;     -   Mn 0.2 to 2.5, preferably 0.4 to 0.8;     -   P less than 0.025; S less than 0.02;     -   Cr less than 2, preferably 0.8 to 1.0;     -   Ti less than 0.03, preferably less than 0.015;     -   Mo less than 0.6, preferably 0.25 to 0.4;     -   Ni less than 0.6, preferably 0.2 to 0.3;     -   Al 0.001 to 0.05, preferably 0.02 to 0.04;     -   V less than 0.5, preferably less than 0.1;     -   Nb less than 0.1, preferably less than 0.06.

Using annealing steels of these compositions and the method according to the invention, tubular products can be provided with notched bar impact work values of at least 70 J/cm² at 293 K and at least 50 J/cm² at 223 K. Such steels are particularly suitable for the production of airbag tubes with a tensile strength of at least 700 MPa, in particular at least 900 MPa.

Finally, protection is claimed for a tubular product, in particular an airbag tubular product, which was produced using the method according to the invention.

Such an airbag tube has a tube wall with an outer face and an inner face, wherein the diameter of the tube wall is reduced at one tube end. At least a partial length with reduced diameter is rolled and formed partially closed, wherein a rotationally symmetrical outlet opening delimits the tube wall at the rolled end face of the tube. The partial length with reduced diameter has a carbon distribution, measured in the wall thickness direction, in a tolerance band of no more than 10 percent, and/or the partial length with reduced diameter is free from over-rolling, flaking, loose particles or chips.

In this way, an improved product surface is achieved with reduced scale formation because of the lower shaping temperature, short process times and the exclusion of air oxygen by constant contact between the tool and the heated surface in the shaping zone.

Also, no flakes or loose particles occur on the product surface, since no over-rolling takes place. To this extent, on later use of the product, for example on triggering of the airbag gas generator, no such particles can become detached and enter the airbag. Furthermore, such particles in the tool may cause persistent pitting on subsequently shaped parts.

Because of the lower shaping temperature, short process times and the exclusion of air oxygen by constant contact between the tool and heated surface in the shaping zone, a product surface is achieved with minimum partial or complete decarburization (soft skin). This partially and/or completely decarburized surface constitutes a weakness for mechanical damage in conventionally produced tubular products, in particular airbag tubular products.

Also, tubular products or airbag tubular products show less strength loss of the material in the shaping zone because of the lower shaping temperature and short process time.

Also, no chips occur on the tubular product since the material removal process is obsolete. The presence of chips in the interior of the generator housing can therefore be safely excluded. In conventional production of airbag tubular products, such chips can be propelled into the airbag when the system is triggered, and hence cause personal injury.

Further objectives, advantages, features and possible applications of the present invention arise from the following description of exemplary embodiments with reference to the drawings. All features described and/or shown in the drawings, alone or in any sensible combination, form the subject of the present invention, even independently of their summary in the claims or back reference.

In the drawings:

FIG. 1 : shows an exemplary embodiment of a steel tube to be provided for production of a tubular product with the method according to the invention,

FIG. 2 : shows the steel tube from FIG. 1 after an inventive method step for production of an uncalibrated pre-geometry,

FIG. 3 : shows the pre-geometry from FIG. 2 after a calibration, and

FIG. 4 : shows the final produced tubular product.

FIG. 1 shows a steel tube 2 which serves as a starting product for production of a tubular product according to the invention using a method according to the invention. The steel tube 2 is here configured as a rotationally symmetrical element with a constant outer diameter 5, which has a rotational axis 10 configured as a longitudinal axis.

After provision of the steel tube 2, this is shaped into a pre-geometry 3, wherein in an end region 4, an outer diameter of the steel tube 2 is reduced by axial movement into an outer tool (not shown here). Such a reduced outer diameter 6 is shown in FIG. 2 , in which the end region 4 has already been shaped accordingly but not yet calibrated. This calibration takes place when the pre-geometry 3 is laid in an outer tool (not shown here), in which an inner mandrel (also not shown here), with an outer diameter corresponding to the inner diameter 7 of the calibrated pre-geometry 3, is introduced into the end region of the pre-geometry 3. Thus the inner diameter 7 of the pre-geometry 3 is calibrated by this shaping.

By the reduction of the outer diameter 5 of the steel tube 2 to the outer diameter 6 of the pre-geometry 3, naturally the wall thickness in the end region 4 of the pre-geometry 3 increases in comparison with the wall thickness in the end region 4 of the steel tube 2.

This increased wall thickness is thinned out again by the calibration described. This thinning-out of the wall thickness in the end region facilitates the rolling (described below) of the end region and makes this reproducible. Such a calibrated pre-geometry 3 is shown in FIG. 3 .

The shaping of the steel tube 2 and pre-geometry 3 described above were all carried out by cold forming or cold drawing. In cold forming or cold drawing, the shaping takes place between room temperature and a temperature lower than 473 K, in particular without preheating and without heated tools. So that the steel tube 20 and pre-geometry 3 can slide on one another and on the inserted tools and mandrels more easily, simple lubricant may be used between the individual parts during shaping.

In order now to shape the pre-geometry into the definitive tubular product 1, the pre-geometry is axially displaced into a drawing tool (not shown in the figures) with a roll-in contour having a U-shaped cavity, with simultaneous shaping of the pre-geometry into the tubular product with a rotationally symmetrical outlet opening positioned centrally in an end face.

In the context of the invention, U-shaped or pot-like means an at least partially curved course of a wall between the outlet opening and the unshaped tube portions, e.g. also hemispherical.

This shaping takes place as hot forming or semi-hot forming between a temperature of 473 K up to an Ac1 temperature of the steel used, which lies at 1173 K for the steels and steel alloys used for the tubular product.

The end region 4 and outlet opening 8 of the tubular product 1 are shaped by insertion of a second inner mandrel (also not shown here), which has an outer contour corresponding to the inner contour of the calibrated end region 4 of the tubular product 1, and axial movement of the second inner mandrel and tubular product into a second outer tool (also not shown here), the inner contour of which corresponds to the outer contour of the calibrated end region 4 of the tubular product 1.

In this final step of shaping by hot forming, during rolling of the end region 4 and subsequent insertion of the second inner mandrel, a transverse wall thickness 11 on the end face is again slightly reduced, wherein the geometry of the outlet opening 8 is optimized at the same time. Simultaneously, the inner radii 12 and outer radii 13 are reduced in the rolling region of the end region 4.

In the exemplary embodiments shown here, the end region 4 of the steel tube 2 is drawn over the length which is necessary for rolling of the pre-geometry 3 and for forming the outlet opening 8. In a further exemplary embodiment (not shown here), the drawing may take place over a larger end region of the starting steel tube 2. The later rolling however does not concern the entire drawn end region, so that the inner and outer radius of the final tubular product in the unrolled end region is reduced accordingly in comparison with the longitudinal extent of the tubular product lying outside the end region 4.

LIST OF REFERENCE SIGNS

-   1 Tubular product -   2 Steel tube -   3 Pre-geometry -   4 End region -   5 Outer diameter -   6 Outer diameter -   7 Inner diameter -   8 Outlet opening -   10 Rotational axis -   11 Thickness -   12 Inner radius -   13 Outer radius 

1. Method for production of a steel tubular product (1), in particular an airbag tubular product, with the following steps: a) provision of a steel tube (2), b) shaping of the steel tube (2) into a pre-geometry (3), wherein in an end region (4), an outer diameter (5) of the steel tube (2) is reduced by axial movement into an outer tool, c) calibration of an inner diameter (7) of the pre-geometry (3), wherein the pre-geometry (3) is still laid in the outer tool, and an inner mandrel, with an outer diameter corresponding to the inner diameter (7) of the calibrated pre-geometry (3), is introduced into the end region (4) of the pre-geometry (3), and the pre-geometry (3) is pressed against the outer tool such that the inner diameter (7) of the pre-geometry (3) is calibrated by shaping, d) removal of the pre-geometry (3) from the outer tool (5) and removal of the inner mandrel from the pre-geometry (3), e) axial movement of the pre-geometry (3) into a drawing tool with a roll-in contour having a pot-like concavity, with simultaneous shaping of the pre-geometry (3) into the tubular product (1) with a rotationally symmetrical outlet opening (8) positioned centrally in the end face, f) removal of the tubular product (1) from the drawing tool.
 2. The method as claimed in claim 1, characterized in that the shaping in step b) takes place as cold forming or cold drawing.
 3. The method as claimed in any of the preceding claims, characterized in that the shaping in the further step takes place as cold forming.
 4. The method as claimed in any of the preceding claims, characterized in that the shaping in step d) takes place as hot forming or semi-hot forming.
 5. The method as claimed in any of the preceding claims, characterized by a finish-forming of the end region (4) and outlet opening (8) of the tubular product (1) by insertion of a second inner mandrel, which has an outer contour corresponding to the inner contour of the end region (4) to be produced of the tubular product (1), and an axial movement of the second inner mandrel and tubular product (1) into a second outer tool, the inner contour of which corresponds to the outer contour of the end region (4) to be produced of the tubular product (1).
 6. The method as claimed in claim 5, characterized in that for calibration or finish-forming of the outlet opening (8), the second inner mandrel is used with a rotationally symmetrical element positioned centrally in the end face, the outer contour of which element corresponds to the inner contour of the outlet opening to be produced of the tubular product (1).
 7. The method as claimed in claim 5 or 6, characterized in that the calibration of the end region (4) and/or the outlet opening (8) takes place in a residual heat of a hot forming or semi-hot forming carried out in step e), in particular at a temperature of at least 473 K or lower than a Ac1 temperature of the used steel or used steel alloy.
 8. The method as claimed in any of the preceding claims, characterized in that before step d) (rolling-in), the tubular product is heated to >Ac3 temperature, and after step d), or in the case of calibration of the outlet opening, is actively cooled so that an at least partially hardened grain structure is formed in the steel alloy.
 9. The method as claimed in any of the preceding claims, characterized in that as steel for the steel tube, a steel alloy and in particular a hardenable ultra-high strength steel is used.
 10. The method as claimed in claim 8, characterized in that as a material of the tube or tubular product to be produced, a steel is used which, as well as iron and unavoidable melt-induced contaminants, comprises the following alloy elements as percentage by weight: C 0.07 to 0.50. preferably 0.07 to 0.20; Si 0.05-0.55; Mn 0.2 to 2.5, preferably 0.4 to 0.8; P less than 0.025; S less than 0.02; Cr less than 2, preferably 0.8 to 1.0; Ti less than 0.03, preferably less than 0.015; Mo less than 0.6, preferably 0.25 to 0.4; Ni less than 0.6, preferably 0.2 to 0.3; Al 0.001 to 0.05, preferably 0.02 to 0.04; V less than 0.5, preferably less than 0.1; Nb less than 0.1, preferably less than 0.06.
 11. The method as claimed in any of the preceding claims, characterized in that the tubular product (1) produced has a tensile strength of 700 MPa, preferably at least 900 MPa.
 12. Tubular product (1), in particular an airbag tubular product, produced according to a method as claimed in any of claims 1 to
 12. 